texts: Can we detect the wobble of neighbouring stars around their barycenters?

texts:
The wobble of a star around its barycenter is interesting (the wobble caused by the pull of the rest of the solar-system on the sun).

This question explains why we don't see this wobble from within our own solar-system (because we are mostly orbiting the sun, and not our solar-system's barycenter):
What point does Earth actually orbit?

But is it possible to detect from here on Earth, the wobble of neighbouring solar-systems relative to the wobble of our solar-system?


texts: How do astronomers detect the 'metals' in a star? If the atoms are presumably completely ionized?

texts:
Atoms and molecules usually emit their characteristic wavelengths because of the electrons' energy levels....
Do the completely ionized ('naked') nuclei absorb and/or emit EM radiation?
If so, at what type of wavelength?  Mostly visible, IR and UV like the rest of the star?


texts: Have we Observed Continuum Emission from Neutron Stars?

texts: Have we detected continuum optical emission from any rotating neutron stars that do not have an accretion disk dominating the light? I ask because I know we have observed Doppler broadening of spectral lines from the rotation of even ordinary stars rotating at ordinary rates, but it seems to me that only the rapid rotation of a neutron star would be fast enough to make the Doppler broadening of the thermal continuum peak to be observable.


texts: What I can observe with a 80/900 refractor telescope? What can I study?

texts:
Recently I bought a telescope after a very good conversation with an astronomer friend of mine. The main goal is to see planets.

However, Mars is so difficult and is far to find it in opposition. I really like the views of the Moon and there is where I spend most of my time observing. But I really want to go view the planets, but I seem a bit without an objective. I don't want only to see them at random; I want to do some kind of study using my observations. So, can you suggest something to do with my telescope to start some serious study?

I have a good base of Physics (I'm a computer engineer) and I really like Math, but I'm a complete amateur in astronomy.


texts: How much solid matter is in the observable Universe?

texts:
I want to know the volumetric ratio of empty space to planets, stars, and all other solid matter combined. Not considering any dark matter, dark energy, or black holes.
For example, you can say X percent of the observable space is just empty space. The remainings are comets, stars, planets, and so on.
Edit:

I meant solid as in all the things that are not gas.
I know that space is not exactly empty, let's consider that empty means all other things except solid.
I'm not looking for exact numbers but just an estimation.


texts: How to calculate the position of the sun in long/lat?

texts:
I have a world map, and I want to know, given a timestamp, where is the sun located right now, in terms of long/lat.

Here is a website that does that:
https://www.timeanddate.com/worldclock/sunearth.html

But they do not share the calculations.

How can I calculate the sun's position in earth gps terms given a timestamp?

Edit: some context

I have a plane that is determined by 2 points on earth, and that plain is perpendicular to the earth in those points. I want to know where is the sun relative to that plane, so I can know if the sun is to the right of it, or to the left of it.

Instead of working in 3 dimensions, I decided to work in 2. I already have that plane in 2D (just a line on the map), now I need to know where the sun is

Edit2: Using the wiki page here, this is what I got so far

Note: The interesting part is getCurrentPosition

class Sun {
    private julianTime(timestamp: number) {
        // Source: http://stackoverflow.com/questions/466321/convert-unix-timestamp-to-julian
        return timestamp / 86400 + 2440587.5;
    }

    private convertEclipticEquatorial(lambda: number, beta: number, R: number, n) {
        const epsilon = 23.439 - 0.0000004 * n; // Obliquity of the ecliptic
        console.log("epsilon", epsilon);
        const alpha = Math.atan2(Degrees.cos(epsilon) * Degrees.sin(lambda), Degrees.cos(lambda));
        console.log("alpha", alpha);
        const delta = Math.asin(Degrees.sin(epsilon) * Degrees.sin(lambda)); // declination
        console.log("delta", delta);
    }

    getCurrentPosition(timestamp: number) {
        const JD = this.julianTime(timestamp);
        console.log("JD", JD);

        // Source: https://en.wikipedia.org/wiki/Position_of_the_Sun
        const n = JD - 2451545;
        console.log("n", n);
        const L = (280.460 + 0.9856474 * n) % 360;
        console.log("L", L);
        const g = (357.528 + 0.9856003 * n) % 360;
        console.log("g", g);
        const lambda = (L + 1.915 * Degrees.sin(g) + 0.020 * Degrees.sin(2 * g)) % 360;
        console.log("lambda", lambda);
        const R = 1.00014 - 0.01671 * Degrees.cos(g) - 0.00014 * Degrees.cos(2 * g); // Distance
        console.log("R", R);

        this.convertEclipticEquatorial(lambda, 0, R, n);
    }
}

class Degrees {
    static sin(angle: number) {
        return Math.sin(angle / 180 * Math.PI);
    }
    static cos(angle: number) {
        return Math.cos(angle / 180 * Math.PI);
    }
}

console.log(new Sun().getCurrentPosition(new Date().getTime() / 1000));

Running this now, gives:

  JD 2457844.130512928 (can confirm, this is correct)

  n 6299.1305129281245 (simple math)

  L 9.181612328272422 (simple math)

  g 85.95292328111373 (simple math)

  lambda 11.09465308568091

  R 0.9990992788207762

  epsilon 23.436480347794827 (can confirm, that is roughly correct)

  alpha 0.17801325468993906

  delta 0.0766106706512887


texts: Does the milky way have relativistic mass against galaxies which are moving away from it at high speeds?

texts:
According to the article cited behind, and to a post here in Astronomy SE , there are galaxies moving away from Milky Way faster than light, even at speeds of 2.3c . According to this article
Can two galaxies move away from each other faster than light

  The two galaxies we've been discussing are not travelling through
  space, it is the space between them that is expanding. Or put in
  another way, they are stationary and all the space around them is
  being stretched out. This is why it doesn't violate the theory of
  relativity, because it is not motion in the traditional sense.

So does this motion between galaxies "in not a traditional sense" count at all to calculate relativistic mass?


texts: Can I use an array of inexpensive cameras as an alternative to a telescope?

texts:
The link below describes how an array of inexpensive cameras can create very detailed images. Would this work for astronomy?

https://graphics.stanford.edu/papers/CameraArray/


texts: Configuration required to tidally heat Earth-like rogue planet

texts:
Could it be possible for Earth-like rogue planet to maintain large bodies of liquid water on the surface without a sun by relying mostly on tidal forces of a moon (or multiple moons).
How big/far/fast would the moon have to be?

What about a binary planet instead of planet and a moon?


texts: Do far away planets reflect light in the same way our moon/earth reflects light?

texts:
Here’s a scenario:

Say you’re up in space and you’re far enough way that you can see the planet earth. Whilst looking down, you can see  blue sky, blue water and green land and white clouds and darker shades of different colors. This is because our sun is reflecting light off our planet that we can see these colors.

Now you’re up in space and you’re far enough way that you can see the planet Jupiter. If you looked, would you be able to see colors in the same way we’ve seen from Juno’s pictures. I see red, white, orange… The sun is so much further away, how can the colors we see be just as bright/visible to us? The same question holds true for planets outside our solar system. They’re so far way from there light source, yet I can see colors on them. How come the distance of the light source doesn’t affect the brightness of the colors I see on these planets?

Like if I was to turn on a light bulb in a huge dark room and shine that light in the middle, the surrounding areas would be much brighter than the furthest point of that room/and at some point there would be no light reaching an even further point.

Bear in mind I’m a basic idiot when it comes to this stuff.


texts: Is it possible to tell the time of day by shadows on the photo

texts:
I have a photo where objects and their shadows (e.g. trees, people) are visible, the Sun is not visible directly, but it is clearly neither dusk nor dawn.

Let's assume that coordinates of this place are known more or less precisely so that I can find it on google maps.

I also know the date when this photo was taken but don't know the time of day. Is it possible to estimate it from the photo?

I have a debate with a friend of mine about this.
My friend states that a reasonable enough estimate is to calculate the Sun's elevation above the horizon by

$$
\text{elevation} = \arctan \frac{\text{object's height}}{\text{shadow's length}}
$$

and then use one the myriad websites on the internets to look up the time for that date and location.

I doubt it because

The length of a shadow on the photograph depends on the relative position of the Sun, the object and the camera.
Most importantly, the slope of the ground can't be estimated neither from the photo, nor from google maps. This picture is a good example.

Who's right? Am I missing something?

Also, it would be interesting to understand the role of uncertainties in this calculation. Assuming that the surface is reasonably smooth and the slope is close to zero, what uncertainties (location, object's height, shadow's length, the slope, camera position) would play the major role? Does it depend on the time of year?


texts:
I understand that - although appearing yellow - the sun is white. And that this is because the other light colours are scattered by the atmosphere. For example:

Google image search for "The Sun from Space"

That was difficult for me to come to terms with.

But if yellow light is what's getting through the atmosphere - why doesn't everything have a yellow tinge to it?


texts: What is the relationship between AGN and [OIII]

texts: I read that we can observe [OIII] from AGNs but what processes in AGNs produces these lines why we can use them to trace or tell if that galaxy is active or not?


texts: Is looking through a telescope to do astronomy a thing of the past?

texts:
Do astronomers actually look through a telescope with the naked eye or do they mostly look the the data generated by telescopes on computers?

Are they mostly working behind the computer and pencil and paper or is it still stargazing?

Do they actually point a red laser to observe constellations or do they do it all on the computer, and lasers that can point us at constellations are more of a gimmick for amateurs?

I’m sure astronomers would need to look under the lens to confirm the computers aren’t processing data that’s wrong. Do astronomers actually trust the algorithms that automatically do data entry from telescope observed data to their notebooks enough that they usually don’t bother to under the telescope until they’ve confirmed something?


texts: How is the difference of bolometric magnitudes not dependent on the stars' radii?

texts:
The difference of 2 bolometric magnitudes is given by:
$$M_{bol, ★} - M_{bol, ☉} = -2.5 \cdot \log \left( \frac{L_★}{L_☉} \right)$$
But Pogson's equation is:
$$M_{bol, ★} - M_{bol, ☉} = -2.5 \cdot \log \left( \frac{F_★}{F_☉} \right)$$
where $F_★=\frac{L_★}{4\pi R^2}$, so how come the first equation isn't dependent on the radius?


texts: What is the "hypothesized lower mass gap" between 2.5 and 5 solar masses?

texts:
Searching here for "mass gap" yields nothing yet. Penn State's Black hole or neutron star? LIGO-Virgo scientists find mystery object in 'mass gap' begins:

When the most massive stars die, they collapse under their own gravity and leave behind black holes; when stars that are a bit less massive than this die, they explode and leave behind dense, dead remnants of stars called neutron stars. For decades, astronomers have been puzzled by a gap in mass that lies between neutron stars and black holes: the heaviest known neutron star is no more than 2.5 times the mass of our sun, or 2.5 solar masses, and the lightest known black hole is about 5 solar masses. The question remained: Does anything lie in this so-called mass gap?

and later says:

"This is going to change how scientists talk about neutron stars and black holes," said co-author Patrick Brady, a professor at the University of Wisconsin, Milwaukee, and the LIGO Scientific Collaboration spokesperson. "The mass gap may in fact not exist at all but may have been due to limitations in observational capabilities. Time and more observations will tell."

The paper is the 23-June-2020 ApJ Letter GW190814: Gravitational Waves from the Coalescence of a 23 Solar Mass Black Hole with a 2.6 Solar Mass Compact Object which says:

GW190814's secondary mass lies in the hypothesized lower mass gap of 2.5–5 M⊙ (Bailyn et al. 1998; Özel et al. 2010; Farr et al. 2011; Özel et al. 2012) between known NSs and BHs.

Question: I'm curious about the use of the word "hypothesized" here. Is said hypothesis that there is  a mass gap at all (versus there not being a mass gap) or is it that the should be a mass gap? In other words, is the observed lower† mass gap strictly an observation, or is it something that at least some predict should be there?
†there's another mass gap starting at about 45 M⊙ related to the upper limit of stellar-mass black holes

In the popular press for example, Forbes published Is LIGO About To Destroy The Theory Of A 'Mass Gap' Between Neutron Stars And Black Holes? but the word "theory" does not appear anywhere else in the article, so I assume there are no such theories.
However it does contain a really interesting statement:

The most recent microlensing studies take advantage of data from the ESA's Gaia mission, and find no evidence at all for this purported mass gap. Instead, they have uncovered a number of interesting microlensing candidates with exactly the masses you'd need to fill in this so-called gap.

Interesting that we go from "The Theory of a Mass Gap" in the beginning, to "this so-called gap" at the end.


texts: How big a rock can hit the sun and it not matter?

texts:
If a rock the size of Oumuamua were to crash into the sun — assume central collision — would it materially affect the sun’s behaviour? Would unaided humans (rather than sun-tastic special telescopes) notice a change in the sun’s brightness or colour or something?

What if the rock were the mass of Pluto? The Moon? The Earth? How big a rock can smite the sun and unaided humans not notice?

Edit: this is not about long-term effects of a merger. This is about the on-the-day or maybe in-the-month effects. How large would a rock need to be for its collision with the sun to have an effect?


texts: Why do constellation names change (eg from Cancer to Cancri) when used as part of a star's name?

texts:
Cancri is what to Cancer?  or Leonis Minoris to Leo Minor?   Arietis to Aries ?  or Lyncis to Lynx.

Are they the Flamesteed names, the Latin names, Greek names?
What is their name?


texts: Does the sky get darker faster during the winter?

texts: When one lives away from the equator, say at around 37° latitude, does the sky get darker faster after the sun sets in the winter as compared to the summer?  Why is this the case?  I believe that if you were to draw the path the sun traverses at different times of the year, the paths are parallel to each other but shifted, such that in summertime, the path is longer with a shorter section below the horizon and in winter it is shorter with a longer section below the horizon. I imagine that the angle of the path relative to the horizon is approximately the complement of your latitude.


texts: What happens if two or three stars collide with each other?

texts:
After finding out that there is a solar system with three stars, I did extensive research. I couldn't get a definitive answer. What would happen if they collided with each other?

I know there would be total destruction everywhere, but I would like to know what, including more scientific short- and long- term effects, would happen?

If you can't explain three stars, two stars colliding would be okay. I'm still an amateur at astronomy, so please don't make the answer too complicated.


texts: What resolution is needed to see a quasar through the galactic center?

texts:
The center of the galaxy is densely packed with stars and obscured by a whole lot of dust between us and it. For those reasons, groups that study the motion of stars around the super-massive black-hole in the center of the Milky way need to use big telescopes at near infrared wavelengths. Questions of dust obscuration aside, what resolution would a telescope need to have to pick out a background quasar looking through the center of the Milky Way?

SDSS QSOs start at around 8th magnitude (Vega) in 2MASS photometry ($H$ and $K_s$ bands), and rise like a power law as flux goes down Here's a quick and dirty plot of SDSS QSO survey made from SDSS data release 10 made using topcat. I would convert the graph to counts per solid angle, but I'm too lazy to look up the survey area of the SDSS spectroscopic survey right now.

There's a quick and dirty example of the effect I'm describing in Figure 2 of the Ghez et al. (2008) paper Rob Jeffries linked to below. The difference between the red points and blue points is both exposure time (more photons) and higher Strehl ratios (a measure of how close to diffraction limited the image is and, hence, a proxy for resolution given the same optical system).

Generically, you could say that the question is: how do we determine the confusion limit in images with point sources? This is just specifically applied to the field of view on the sky with the highest density of resolvable point sources available. Concretely, say we wanted to resolve $22^{\mathrm{nd}}$ magnitude sources in views of the galactic center, what resolution is required to do so?

Getting to the full answer to the more concrete question "how deep would we have to look to see a background quasar" would require additional information: size of field of view, and dust extinction along the sight line. So, for simplicity, I'm assuming that if you're resolving $22^{\mathrm{nd}}$ magnitude sources in $K$ band, you're probably seeing a quasar. Thus we only need the projected density on the sky of sources brighter than that cutoff, roughly, and some description of how resolved sources need to be from the background brightness to detect them to answer the question.


texts: Why doesn't the Moonrise appear red when viewed from orbit?

texts:
After searching through many videos, I finally found one that shows, from the perspective of low earth orbit, the Moon transiting the horizon. It is actually a Moonset, but it shouldn't make a difference.

The Moon does not appear to change its color, neither gold nor red.

This contrasts with my understanding of atmospheric effects. Here on Earth, the Moon does appear more yellow (and when halfway or less above the horizon, even red) during Moonrise. The atmosphere tends to scatter blue light, leaving only yellow and red behind.

So why isn't this happening in the video? From orbit, we should be seeing the Moon through even more atmosphere. But I can't detect a trace of red or yellow in the Moon.

Granted it's not the best quality video that it could be. It could be more zoomed in. But it was the only video I could find that showed the Moon transiting the horizon from space.


texts: Observing the motion of the solar system and galaxy through space?

texts:
This is very perplexing.
Conditions:

planets are moving around the sun (solar system)
solar system is moving as part of the galaxy
the galaxy is moving through space on some axis
galaxy is part of a cluster, also moving through space
other stars and constellations are also moving through space, presumably in the same direction and speed as the Milky way.

I appreciate the answer to Why is the Solar Helical (Vortex) model wrong? - but considering the question: how can we observe the solar system's Helical motion through space, I have a new question... please allow me to present both, since they are related.
Question 1:
Again, how can we observe the Helical motion of our solar system (and galaxy) from earth? Is it by observing the passing of planets across/through constellations?
Question 2:
If I can make an analogy: like different ticking clocks stored inside a grandfather clock - is the solar system following its own Helical motion and therefore are all other solar systems within the Milkyway also following their own Helical motion? AND is the galaxy also following its own Helical motion!?
OR!
Is the solar system's motion fixed (Helically speaking), and any such movement is tied to the motion of the parent, i.e. the galaxy? In other words, is the Helical motion of the solar system the same as the galaxy, or are all solar systems moving Helically through space, e.g. many clocks within a grandfather clocks analogy.
Initially, the question was simple: how can be observe the solar system's Helical motion?
But it seemed unintuitive to me that the solar system is moving Helically while the galaxy is also moving as such - both structures on their own trajectory and paths - seems like galactic chaos.
And, all the while planets maintain their fixed orbits.
I would have guessed that the solar system is NOT moving Helically, but the galaxy is, and the solar system is simply a fixed orbital passenger, as so to speak - in the same way 8 grains of sand (planets) are passengers in a bowl of jelly (galaxy), both being carried by another vehicle (cluster).
It seems to me then, the solar system's motion through space is limited by the galaxy's motion, in terms of axis, speed - and therefore the galaxy is also fixed like a passenger on a much larger galaxy cluster.
It seems reasonable to suggest the whole universe is moving Helically(?), and all galaxies merely inherit this, but are more like grains of sand in jelly than independently ticking clocks within a larger grandfather clock.
If our solar system is moving Helically independently from the galaxy, the solar system could move at its own speed, either quicker or slower depending on environmental factors. Could the solar system really skip-out of the galaxy, racing ahead on its own Helical path(?).
So many questions! But Question 1 is the one I wish to request help answering.


texts: How (un)stable are the Lagrangian points 1, 2 and 3?

texts:
A couple questions, please: I know that the Lagrangian points 1, 2 and 3 are unstable and special Lissajous orbits plus some station-keeping are required to place a spacecraft around them. But I was wondering if they are so totally unstable that they can't temporarily "capture" a passing natural object (let's say an asteroid or a cloud of cosmic dust, in a stationary way or entering some kind of not-very-stable orbit around them)? Or could they?

And if they were able to, how much time would (approximately) be required for this/these object(s) to go "off-orbit" and abandon the L(1,2,3) area? In the case of the cloud of cosmic dust, would it just slowly (or quickly) drift away, or would they be "launched" towards another direction or orbit?

Thanks in advance!


texts: What is the exact position of the Large Magellanic Cloud?

texts:
I am not able to visualize the position of the LMC and have not found a satisfactory explanation...
Can someone please answer it this way?

Relative to a ray joining the Earth to the Galactic-Center(Milky Way's), please describe the position in the plane of the ray as- to the South/East/West or their combinations. And will the galaxy be below/above the plane of the ray?

Note: Please consider the direction of the ray(Earth->Center) as the axis of a compass that points towards the north.
E.g://Answer format that i was hoping for:
   To the South-West of the Segment, and below its plane.


texts: Saw 8 stars moving in same direction one by one

texts: I saw 8 stars moving in the same direction one by one and then at the same point they faded or you can say disappear. The speed was not too fast but not too slow either. I am more curious because there were 8 of them and one by one they came from the same direction, move in the same direction and disappear at the same place. What could it be satellites or were the actually stars?


texts: Is it possible for 'pre-big bang' empty space to have the property of being able to spawn matter and/or energy into existence?

texts: .. and if so, wouldn't this also mean that during this event, time would be created as a collateral, as a canvas for this property to manifest?


texts: Can galaxy formation process explain Tully-Fisher relation in dark matter theory?

texts: I read that, unlike MOND theory, Tully-Fisher relation is believed to result from galaxy formation in dark matter theory. What is the exact current status regarding the explanation of Tully-Fisher relation from galaxy formation process?


texts: I've studied that the major axis of a planets orbit = ( radius of perihelion + radius of aphelion) . But then  how can we calculate the value of the minor axis if we are provided with the values of radius of perihelion and radius of aphelion only ?


texts: What is the meaning tiny bundle of energy smaller than an atom

texts: At the beginning of a video on youtube titled "history of the world in 2 hours", it is said that "every thing begins with this tiny bundle of energy smaller than an atom". I want to know what it means.  atom is matter.  energy is different from matter and is measured in joules where as size is measured in volume units. How can one say energy smaller than an atom?


texts: How are exoplanets with 10,000 day periods discovered with the radial velocity method?

texts:
The radial velocity technique has apparently been used to discover exoplanets with extremely long orbital periods of ~10,000 days. For example, see this link:

https://exoplanetarchive.ipac.caltech.edu/cgi-bin/TblView/nph-tblView?app=ExoTbls&config=planets

If the period of a planet is ~10,000 days, then the period of the star must be the same. However, this would mean that observations would need to be made over ~30 years to get a complete picture of the radial velocity variation of the star, a time which is far too long. How then are radial velocity measurements used to discover such planets?


texts: An exoplanet with density 0.31 grams per cubic centimeter has been found. Is this the least dense exoplanet we know of?


texts:
I understand that a black hole happens after a star exhaustions elements capable of undergoing fusion.  Without the energy produced by fusion the remaining mass of the star collapses on itself and forms a black hole.  I conclude black holes must be deficient in hydrogen, helium and elements that undergo fusion in stars.  I understand that hydrogen and the like undergoes fusion in stars in part (or completely?) because of the gravitational pressure which occurs in stars.

Suppose a young star replete with hydrogen merged with a black hole.  My question: would the result be a larger black hole, or a radiant star with a very massive heart?  Could a massive infusion of (fusible?) elements reboot the black hole back into a star?


texts: How to compute the perihelion precession of Earth's orbit?

texts:
I want to simulate the perihelion precession of Earth. I tried searching  other posts in stack exchange as well relating this question but may be it's my fault that I couldn't understand their answers. I read the history about mathematicians and physicists trying to find a good theory accounting the precession. I read that there are many planetary equation by Newton,Laplace,Hill,Poincare etc. But I am not sure which one gives the more accurate result. I am not even sure if I going on a right track searching the correct theory.
I'd really appreciate if anyone could help me with the theory which gives fast and more accurate result?


texts: What would Jupiter look like from a Galilean moon?

texts:
If I were on a Galilean moon of Jupiter, like Europa or Ganymede, what would the planet look like? (Let's say I'm on a space station that provides atmosphere to make it slightly less unrealistic.)

Would it look like our moon, only a bit bigger and colored like Jupiter, with the stripes and the "eye", or would the moons be close enough for Jupiter to look gigantic and fill a significant portion of the sky?

The two moons that are closest to each other are Io and Europa. They seem to be about 250,000 kilometers away from each other. Would it be possible to see one from the other with the naked eye if they were aligned? For example, if Europa were in a position in its axis where it is lit by the sun, could I see it, looking at it from Io? Or would I need a telescope? A 3000 km sphere from 250,000 km away does not seem possible. The other way around, of course, Io is a bit bigger.

But the part I'm the most interested in is: what would Jupiter look like from one of these moons.


texts: Is it true that stars start with a rocky core, then collect hydrogen by the gravity of that rocky core, just like planets collecting materials?


texts: How easy would be to see the Moon from Mars?

texts:
This question arises from "What planet is better than earth to infer solar system configuration?", where one of the answers pointed that it's easier to discover the Moon from Mars than discovering the Galilean satellites of Jupiter from Earth.

Then my question is:

How easy (or difficult) would be to see the Moon from Mars?
Could it be seen with naked eye?
Is actually easier to see the Moon from Mars than to see the Galilean moons of Jupiter from Earth?

I know there are favourable factors (Mars is closer to Earth than Earth to Jupiter, and Earth is smaller than Jupiter, so it's expected to obscure less its moon than Jupiter obscures its) and also unfavourable factors (the Moon is closer to the Earth than the Galilean moons to Jupiter, and Earth and Moon are backlit when seen from Mars). What is the overall result?


texts:
Dark Matter's observational evidence is widely attributed to Vera Rubin based on her observation of galaxy rotation curves with Kent Ford in 1970s, which was later confirmed by Albert Bosma in 1978.

However, it was in the late 1930s that Fritz Zwicky inferred the presence of dark matter based on his observations of Coma galaxy cluster using the virial theorem. Herein lies my confusion. Why does the scientific community and people, in general, give credits to Vera Rubin instead of Zwicky for inferring/observing the evidence for dark matter? Or, is the recognition given because she independently strengthened Zwicky's observation thereby confirming the existence of dark matter?


texts:
I am doing a homework on globular clusters and could not find trustworthy data on the average diameter of a globular cluster. Most clusters I included had a diameter of 120-150 ly.
But is that the average diameter of such globular clusters?


texts:
I was going through the derivation of Kepler's 1st Law in the textbook "An Introduction to Modern Astrophysics" by Carroll and Ostlie. From there, I got stuck in a few places in their derivation as I found it confusing. In total, I have 2 questions.

Q1: In the second sentence that I highlighted, it is mentioned that when equation 29 is translated into a physical standpoint, it implies that "both objects in a binary orbit move about the centre of mass in ellipses, with the centre of mass occupying one focus of each ellipse" however I don't see how the physical implications of equation 29 will translate into this that they claim. This equation clearly only describes the position vector of the reduced mass, so how can it be extrapolated to talk about the motion of a binary system when there are 2 masses rather than 1 reduced mass. (Clarification: I am aware that reduced mass is a way to model binary systems, but in this case I don't see the link between the motion of the reduced mass and the motion of 2 bodies)
Q2: On the very last line, it is mentioned that "L is at minimum as eccentricity approaches unity, as expected". However I don't see the qualitative reasoning as to why this would be "expected". Is there a physical or physics explanation as to why we expect an orbit with a large eccentricity to have a lower angular momentum than one with a low eccentricity?


texts: Calculate the true diameter of stars from photographic plate

texts:
I am reading the book "Fourier Analysis" from T. W. Körner, and in chapter 95, he explains how you can compute the diameter of stars on a photographic plate. He says that "Since observations of the nearest stars at six-monthly intervals (i.e. using a diameter of the earth's orbit as a surveyor's base line) enable astronomers to measure the distance of these stars, knowledge of the apparent diameter (i.e. the diameters of the discs on the photographic plate) would then enable us to calculate the true diameters of the nearest stars".

So how do you calculate the true diameter of a star given its apparent diameter on a photographic plate? Is it related to the "zooming capacity" of the telescope one uses?

And if for instance, using a 100'' telescope, you measure that a star has an apparent diameter of 0.66mm, can you directly compute the true diameter of the star?


texts: For what parameter does the abbreviation MA stand for?

texts:
In the sentence:
"We added a Keplerian model with P = 120.713 days, m = 14.629M⊕, e = 0.0448, ω = 251.36°, and MA=216.88° plus Gaussian noise with rms=5.9696 ms-1."

P is the orbital period, e the eccentricity,... But what is MA? I'm guessing its the mean anomaly, but the mean anomaly is a time-dependent variable which (for my understanding) does not make sense in this context...
Quote is from https://iopscience.iop.org/article/10.1088/0004-637X/719/1/890 (page 24).


texts: Is there a MATLAB equivalent of PyEphem?

texts: Many people on the Astronomy stack exchange use the PyEphem  package for the Python  programmimg language to perform astronomical computations.  Are the underlying numerical routines (from XEphem) also available for MATLAB?


texts: Does a photon need to have EXACTLY the right energy to be absorbed by a gas molecule?

texts:
From an answer to this question, https://physics.stackexchange.com/questions/281660/how-does-an-electron-absorb-or-emit-light,

Absorption of a photon will occur only when the quantum energy of the photon precisely matches the energy gap between the initial and
final states of the system. (the atom or a molecule as a whole)
i.e., by the absorption of a photon, the system could access to some
higher permissible quantum mechanical energy state. If there is no
pair of energy states such that the photon energy can elevate the
system from the lower to the upper energy state, then the matter will
be transparent to that radiation.

Given that the photon's energy is proportional to the photon's electromagnetic frequency, and that the frequency is subject to tiny Doppler shifts due to differences in velocity between the emitting and absorbing molecules, how can a photon have precisely the same energy as the exact quantum state change for a gas molecule?  Does the photon energy only have to be very close to the quantum state change for a molecule to absorb it?  If so, what happens to the extra(or lesser) energy absorbed by the molecule? Does this margin contribute to the width of observed absorption spectra in radio astronomy?


texts: Are the stars seen at the equator different throughout the year?

texts:
If you live on the equator and look up at the night's sky at, say, midnight, would you see a completely different sky on the 1st of January than you would on the 1st of July?

I assume you would as the night side would be facing away from the sun by 180 degrees and necessarily looking outward from the solar system 180 degrees from each of the two points at those times.

Is that correct?
Why?


texts: Why is the exoplanet HD 131399Ab so hot?

texts:
Recently, a fascinating triple-star system with an exoplanet has been discovered (arxiv).

I'm very surprised about the temperature of the exoplanet: it is roughly 850 kelvin, even though its orbit is ~82 AU away from the main star (~1.5 $M_☉$), and >250 AU from the other two stars. The planet itself has ~4 jupiter-masses.

Why can such a planet be so hot, for instance compared to Jupiter? I would have expected it to be very cold because it loses a lot of energy via radiation (that's actually the reason why the group discovered it via direct imaging), and it does not get much energy from its stars.


texts: How does the universe create more universe ( expand ) out of nothing?

texts:
By the law of conversation of energy, mass and matter, how does the universe get the energy to expand itself, and also expand faster than light!!
Edit :
my main question is that if the matter, energy and mass in universe is constant how does universe keep creating more of it and why? How can something be generated from nothing if the total energy in the universe can't be destroyed nor created?


texts: How are shockwaves in a star that cause supernova created

texts: I know that during the collapsing of a star 8 times the mass of the Sun at the end of its life, a shockwave is generated that blows the star up. However, I do not understand how it is exactly generated. I would appreciate an answer.


texts: How to use spectral flux density data while using Planck's law giving spectral radiance?

texts:
I'm currently working on a fun project in my free time where I'm trying to calculate the temperature, among other things, of stars based on their spectra. Since I have essentially no prior experience in astronomy, except for quite a strong interest, this may sound like quite a dumb question, but I would appreciate an answer. The data provided from SDSS (Sloan Digital Sky Survey) that I'm using plots the spectral flux density, Fλ, over wavelength, λ. Now my problem is, I'm trying to do a curve fit in Python using Planck's law, (I'm aware that it is also possible to use the proportions between red and blue light, but I first want to see how close I can get using this method) but Planck's law (as below) gives spectral radiance. This means that while the data I'm using is in different units than the numbers I get from Planck's law. Therefore, (as you can see below) they are on greatly different scales. So my question is, how do I fix this? How can I use Planck's law to be able to fit the data? Is there any relationship between units that I have not been able to find? I want to clarify that I have tried my best to find the answer on my own, using Wikipedia and other sources, but my attempts have, obviously, not been rather successful.

The data that I'm using (The y-axis is 10^-17 erg/s/cm^2/Å):

The graphs I have got using Planck's law:


texts: What is the best database for identification of spectral lines?

texts:
What is the best database for the identification of spectral lines? For instance, I have a red spectrum of A0 star and I would like to recognize the most prominent lines, where to find them? Many thanks.
EDIT:
Why is O2 line not in NIST? It would be about 2676.99 A.

source: http://www.spektros.de/sirius/sirius.html
After advice:
How to install the database for molecules https://bitbucket.org/OES_muni/massiveoes/src/master/? I obtained the following error:
python setup.py develop
/home/linux/.local/lib/python2.7/site-packages/setuptools/dist.py:474: UserWarning: Normalizing '1.001' to '1.1'
  normalized_version,
running develop
error: can't create or remove files in install directory

The following error occurred while trying to add or remove files in the
installation directory:

    [Errno 13] Permission denied: '/usr/local/lib/python2.7/dist-packages/test-easy-install-547.write-test'

The installation directory you specified (via --install-dir, --prefix, or
the distutils default setting) was:

    /usr/local/lib/python2.7/dist-packages/

Perhaps your account does not have write access to this directory?  If the
installation directory is a system-owned directory, you may need to sign in
as the administrator or "root" account.  If you do not have administrative
access to this machine, you may wish to choose a different installation
directory, preferably one that is listed in your PYTHONPATH environment
variable.

For information on other options, you may wish to consult the
documentation at:

  https://setuptools.readthedocs.io/en/latest/easy_install.html

Please make the appropriate changes for your system and try again.


texts: How can you determine the initial volume of a planet's atmosphere?

texts:
Since the surface pressure of a planet is determined by the mass of the column of gasses above it one would surmise that to determine the pressure you must know the volume and mass of the atmosphere.

Knowing the composition and volume of the gasses in the atmosphere would allow you to calculate their mass but how do we determine what that volume is?

Assuming we are only concerned with initial conditions when the primary atmosphere (created through accretion) was formed would the volume be arbitrary?

I hope this isn't too broad of a question. I'm trying to pin down a possible initial state that would be created when a new planet is formed and the conditions stabilize.


texts: Precise meaning of collimation/alignment of a binocular

texts:
My question is about what exactly is meant by a binocular being collimated/aligned. I'm familiar with optics in general, but not with the nitty gritty of binocular design.
There are various resources online (here is one example) that describe different methods to align binoculars (sometimes also called collimation but I will avoid this term here to prevent confusion), usually without explaining in detail what alignment means. One might think (perhaps naively) that alignment means making it so that the optical axes of the left and right "telescopes" extending beyond the objectives are parallel, i.e. both "telescopes" are pointing in the same direction. But I'm not sure if this is the case for reasons I'm about to describe.
I currently have two binoculars, one is a Celestron 8x56 and the other is a new Oberwerk LW 12x60. With both binoculars, the fields of view (FoV) I get through the left and right "telescopes" of the binocular are considerably different: namely there is a horizontal shift. This is not due to parallax because even when an object at infinity (e.g. a star) is at the center of the left eye FoV, it is to the left of the center of the right eye FoV. This tells me that the optical axes of the left and right "telescopes" extending beyond the objectives are not exactly parallel, i.e. they are pointing in slightly different directions. Am I right? If so, why aren't the optical axes parallel?
Various resources on aligning binoculars say such a shift is "normal", but without explaining exactly how much shift is "normal" and why (I understand the brain is capable of merging the two images even in the presence of an undesirable shift). But why should there be such a shift at all in a perfectly aligned binocular? Is this always an unintentional consequence of the left and right components becoming misaligned over time, or is the non-zero angle between the optical axes a design parameter? The former would mean both of my binoculars (including the new Oberwerk) are significantly misaligned, while the latter possibility I also find unlikely because when looking at an object at infinity without a binocular, the "optical axes" of the eyes (each intersecting the retina at the fovea) should be parallel.
Any insights are appreciated.
Edit: Regarding how much of a "shift" I'm talking about: I have not tried to quantify it yet. I will try to estimate the shift when I have time later. But it is small enough not to cause "double vision" when looking at a star, and large enough that it is very easily seen by closing one eye and then the other when looking at a star. I thought this horizontal divergence of the optical axes might be intentional since I see it with both of my binoculars (with no appreciable vertical divergence), and at least one manufacturer (Oberwerk) makes a big deal of how well they collimate their binoculars. But I'm not sure.


texts: How to interpret error of absolute magnitude of a star given data from wikipedia?

texts:
This Wikipedia page contains information about the star Spica in the rightmost table. A small snapshot of this table is below.

In this table, the absolute magnitude is given as −3.55 (−3.5/−1.5). Instead of seeing the value and error given as value ± error, I am thinking the error is given as it is because of absolute magnitude is given on a logarithmic scale (as opposed to linear scale). But I am not quite sure how to interpret this as an error reading. How does one interpret this reading?


texts: Why do red galaxies tend to be more massive than blue galaxies? Do we have an explanation for this observation?


texts: Is the universe older than 13.7 Billion years?

texts: I came to know that the cosmic horizon is the part of universe beyond which we cannot see. So if the light from 13.7 billion years ago is what we can observe, does that mean the universe is older than that and we can't see it?


texts: Would a spaceship have to evade interstellar clouds?

texts:
Apparently, there are clouds of "dust" between the stars. Would a starship have to fly around those clouds, trying to find "tunnels" between clouds, or are the interstellar clouds harmless for a spaceship?

I'm mainly thinking in terms of abrasion or (micro)collisions, not radiation, but would welcome information on the latter also.


texts: What determines if a comet's debris will produce periodic meteor showers?

texts:
Background
I was wondering if the debris from the comet NEOWISE that recently passed Earth would provide meteor showers in the future.
I looked up that the Perseid meteor showers happen when the Earth passes through the debris left behind by comet Swift-Tuttle which has a nucleus diameter of 26 km. In comparison, the comet NEOWISE has a nucleus diameter of 5 km. I am guessing this means it is too small to have a debris trail that produces periodic meteor showers? I was not able to find data about the length of these comets' tails.
The Question
What determines if the debris from a comet will produce periodic meteor showers in the sky?
After thinking about it a bit myself it seems like these variables should be important:

Diameter of the comet
Length of tail when comet reaches Earth's orbit (which probably depends on comet size and composition)
Orbit of comet and its relation to Earth's orbit
Orientation of comet / comet's tail in relation to Earth when it crosses the Earth's orbit
Whether or not tail extends to Earth as the comet moves farther away from Sun
Whether or not the debris persists after the comet has left

I was hoping someone could provide more insight into what determines if periodic showers will be produced by a comet. Specifically, what variables are involved (comet size, length of tail, comet's orbit in relation to Earth's orbit, etc.). I'm particularly interested in what determines the length of a comet's tail as well as whether a debris trail persists long after the comet has left.


texts: Equation to find distance between objective and eyepiece

texts: I have a home made astronomical telescope with 100 cm objective lens and 5 cm eyepiece.when viewing an object in infinity the distance between eyepiece and objective is 105 cm. So how could I calculate distance between objective and eyepiece for and far away object.For example a planet 1 AU from earth how much should be the distance between objective and eyepiece ?


texts: What would a gas giant consisting of only earths atmosphere look like?

texts:
Putting aside how such a planet could be formed and just lets assume it exists.

What would an gas giant "look" like if it just consisted of the gas compound earths atmosphere consists of.

Could we just watch through it?
Would we see a burning core and therefore see something like a dim star?
Or would there be still causes that would block vision, even if there were no other components included?

And would such a planet be able to hold it self together?


texts: Can you translate a GPS coordinate to a Galactic coordinate?

texts:
I am a complete newbie to astronomy and all the related.
So if i say something stupid i am sorry.

But i was wondering if it is possible to translate GPS coordinates like the following:

lat: 41.420042122273024
long: 2.1533203149999736

to its corresponding Galactic coordinates?

I am grateful for all insights that you may offer me :)

PS: if this is the wrong exchange to ask this could you refer me to the right place to ask this question.


texts: The observation of changes barycenter of the solar system

texts:
Is it possible to observation changes in the barycenter of the solar system?

There are graphics showing changes barycenter but they are made on the basis of computer simulation (E.g.: here and here).

It is possible? If so, how?


texts: How to place an observer in a cosmological simulation

texts:
Suppose I have a cosmological simulation with points in xyz space at a certain redshift in a cube like this:

And I have these for many cubes of different redshifts say from z=0 to z=2. Now I want to generate how that would look to an observer at z=0. In theory, it should look something like this (from SDSS):

Is there any software that creates mock surveys like this? My goal is not just to have hydrodynamic simulations but rather mock catalogs that are similar to the SDSS survey. I found the following paper: https://www.giss.nasa.gov/staff/mway/cluster/cole_mnras_300_945.pdf which seems to be mostly what I'm looking for but I can't find any software for this online (it seems this paper was written in the late 90s).
EDIT:
I found the following: https://nbodykit.readthedocs.io/en/latest/catalogs/mock-data.html and it might be what I’m looking for. I will update if this is the case


texts: Why can I see the Andromeda Galaxy despite Milky Way's rotation?

texts:
Okay ... so, I woke up from a dream last night wherein I was (apparently) attempting to devise a time-keeping system for interstellar travel (similar to the stardate system used in Star Trek). In the dream, I was attempting to correlate our current calendar, based on Earth's movements relative to the sun, with this fictitious calendar using the positions of intergalactic stellar bodies in comparison to Milky Way. (i.e., What if we could tell time based on the way other galaxies moved in relation to our own galaxy?)

So, in the dream, I was understanding that Milky Way is a spiral galaxy which rotates around a galactic center. That Earth is toward the middle-end of one arm of Milky Way's spirals. That almost every star we see in the night sky is inside Milky Way with us, and that the galaxy Andromeda is a great distance beyond Milky Way and can be seen in the night sky over Earth in the same location (between constellations Cassiopeia and Pegasus) every night since (at least) 10th century C.E.

Assuming Milky Way was graphed on a flat sheet of paper with the Galactic Center plotted at X=0,Y=0, and the arm which contains Earth was located at X=0,Y=-δ, and also assuming the galaxy Andromeda can be seen in earth's sky from this position ... why then, when Milky Way rotates around it's galactic center so that the arm which contains Earth is located at X=+δ,Y=0 (a rotation of 90°), or even X=0,Y=+δ (a rotation of 180°) is the galaxy Andromeda still visible in the same location in the sky?

Unless the galaxy Andromeda is in some sort of galactic-synchronous orbit with Milky Way (which seems unlikely since most of the posts referencing Andromeda here deal with the collision of the two galaxies), then, in the in either of the second rotational positions listed above, Andromeda should no longer be visible.

What am I missing? Why does Andromeda, which is not a part of Milky Way, only move in the night sky in relation to Earth's seasons and not in relation to any other galactic movement?


texts: Find constellation over Earth coordinates on a specific date-time

texts:
I would like some help in finding the exact constellations, or some sort of visualization of the stars over an specific location an time. The idea is to find out the closest star or group of stars on the vertical of that position in that exact moment.

I'm also aware than "vertical" can have some different interpretations, but a nice approximation should be good enough.

I'm not an expert in physics nor astronomy, but I think providing the coordinates and the date/time should be enough for an approximation.
According to google maps, the coordinates are:

39.467062 -0.377381

No idea what format that is, I'm familiarized with degree/minutes/seconds but I don't know exactly how to translate those. The time would be:

  March 25, 2015, 09:40 (Spain)

, which would be 08:40 in UTC, correct me if I'm wrong.

Any help would be appreciated, if you can't provide a direct answer, at least point me to some website form or program (Linux preferably) I can use to find this out.

In case someone is wandering, this involves a girl and a tattoo, so please bear in mind this is something I'm going to be carrying for the rest of my life :)

Thanks!


texts: The recent results on Hubble constant measurements

texts:
In recent news there is this announcement
In the introduction they say:

Distance measurement discrepancy: a $4.4σ$ tension on the value of $H_0$

and I understand the discrepancy is with the value derived using the cosmic microwave radiation (CMB). There will be more publications to follow.
These measurements are local, i.e. of the universe as it is now.
By coincidence I was looking up baryogenesis and came upon the Sakharov condition  for generating the asymmetry between baryons and antibaryons which must have happened by the time the CMB map solidified.

Any of the three necessary conditions that a baryon-generating interaction must satisfy to produce matter and antimatter at different rates: (i) baryon number violation; (ii) C-symmetry and CP-symmetry violation; (iii) interactions out of thermal equilibrium.

iii) seems to me to be relevant to a measurement of the Hubble constant using the CMB data because it would affect what is presumed in the standard cosmology :

The CMB is also used to determine the Hubble constant, where the temperature is analyzed as a function of frequency – a power spectrum – and a best fit analysis is made to constrain the Hubble constant.

If interactions out of thermal equilibrium are responsible for baryogenesis, maybe the assumptions entering the fit should change, and maybe the discrepancy between the two methods tells something about baryogenesis.
As  a particle physicist I am not up to astrophysical calculations. Is anybody familiar with this topic who could say  whether this is a possible suggestion?
Edit:added
This link says that the Sakharov conditions are necessary, and the standard model obeys all three:

During a first order electroweak phase transition bubbles of the broken vacuum form in an unbroken phase. The expansion of these bubble walls is shown to lead to considerable departure from thermal equilibrium.
Since Sakharov’s conditions are satisfied, the baryon asymmetry of the Universe could be created during the electroweak phase transition. Unfortunately, it has been shown that the amount of CP violation coupled with the strength of a first order phase transition is not sufficient to create enough baryon asymmetry in the SM

Surely the standard model must be  used in estimating by fits to the CMB the Hubble constant, but not the extra baryogenesis mechanism necessary for the observed asymmetry. These could affect the thermal equilibrium estimates.


texts:
I am so sorry for being ambiguous. I'm not an astronomer and have have never actually even bothered to look at the sky before. I don't have a telescope nor camera.

Yet, I'm really really curious about what it is I saw in the sky tonight.

Towards, the east I first noticed a "constellation" the looked much like what is described to be the big dipper (It is possible that it could have been Andromeda and Pegasus). It was squarish and had somewhat of a ladle towards it's north eastern corner. The most distinguishable feature of the structure was that it had 3 stars in a straight line. It was the straightest line in the entire night sky.

About a hand's length to the west was another "constellation", my guts said it's name was Orion. Again, I have never seen, let alone distinguished, a constellation properly in my life. It was like a triangle with it's nose pointing towards the east. The sides that converged to the nose seemed to extend a bit backwards like antennas.

And to the west (a few inches north to the Orion was this strange glow.
At first I thought it was my eyes playing tricks on me as sometimes whenever you stare into light bulb or some other light source, a faint glow in that area of your vision often persists. And as I looked directly at it, it slowly faded away into darkness and all that was distinguishable was an extremely tiny star or two. But whenever I looked to something near it (like a tree or constellation) , the patch reappeared. Then, I noticed that there was cluster of tiny little stars. I couldn't make out much. What caused that light patch? It seemed to glow in and out like it was breathing. I am sure that my eyes weren't playing tricks on me because even as I turned around and tilted my head and jumped up and down in the process, the patch remained in the same spot in the sky.

The location which I observed this is from Ernakulam, Kerala, India. The nearest current sky map I could find is the following from Chennai (which is really far away):

I can't make head or tail of it. Please help and no I'm not crazy. I know it sounds like one of those crazy UFO stories but I assure you it's not.

Is there anything like what I've said observed in the night sky before?


texts: How are the photos of the Milky Way taken?

texts: We are inside the Milky Way, right? So, how are we able to take photos of the Milky Way?


texts: Using altitude, azimuth to determine a baseline vector?

texts:
I know the equatorial coordinates of a “beam” pointing from an Earth-based observer toward some distant star. I have converted those coordinates to altazimuth coordinates for that observer and a second observer elsewhere on earth.
Now, I’d like to turn both pairs of coordinates into vectors so that I can subtract them to determine a vector between the two observers. Is there a formula for this? I’ve only found this:
\begin{align}
z &= \sin(alt)\\
hyp & = \cos(alt)\\
y & = hyp \cos(az)\\
x & = hyp \sin(az)\\
vec & = (x,y,z)\\
\end{align}
But I’m not sure if this will actually work. Would they both be in the same coordinate system so that I can, in fact, subtract them to determine a vector between the two points on Earth? Is there an easier/better way to do this? In the end, I need to determine the angle between the “baseline” vector (the vector between the two observers) and the vector pointing from one observer to the star.


texts: Why does the solar wind switch to flowing straight by the time it reaches Earth?

texts:
Between 02:01 and 02:29 the new NASA Goddard video 5 New Discoveries from NASA's Parker Solar Probe explains

finding # 4, A Breaking Point:
Before parker, scientists knew that the corona rotates with the visible surface below it. But they didn’t know how — or where — the solar wind switched to flowing straight by the time it reaches Earth.
Parker has finally spotted signs of this transition — and the changeover happens significantly farther out than expected.

Naively I'd apply conservation of angular momentum and assume that the particles are in individual ballistic orbits, but the solar wind contains charged particles as well as neutrals and so even though of low density and long mean free paths they can interact collectively.
But I don't understand why the solar wind would suddenly stop rotating beyond some radius from the Sun well inside Earth's orbit, rather than beyond the heliopause.
Why does this happen?


texts: I am on the west coast of the U.S. and for several weeks I have seen two large, twinkling objects in the eastern sky at night with the naked eye. In between them there are 3 smaller ones that are stacked vertically and very close to one another. All 5 rise in the sky as a group as the evening progresses. Could this be Orion? Thank you.


texts: Why are the wings of many strong lines, including hydrogen lines and most metal lines, stronger in Dwarfs than in Supergiants?

texts: What is the reason? Is it because Dwarfs have higher surface gravity (log g), in turn related to pressure, than Supergiants so there is more contribution from van der Waals and Stark broadening?


texts: Buying a telescope for the first time. How important is the focal ratio and is a computerised telescope worth it?

texts:
I'm wanting to purchase a telescope for the first time. I've narrowed it down to 2 options based on what I've discovered from researching and my price range. I mainly want do planetary observations and some deep space. The 2 options that I've found are the following

Celestron 114 LCM Computerized Telescope
Orion 9851 SpaceProbe 130 EQ Reflector Telescope

From what I've gathered, both do have decent specs for what I want to do but I'm still a bit lost when it comes to the aperture and focal length. The Celestron 114LCM has a 1000mm focal length but the aperture is only 114mm where the Orion 130 has a 900mm focal length but a 130mm aperture. Am I right in thinking that the Orion telescope would be better for me? Also, with the Celestron telescope it is computerised so, although the fun would be taken out of messing around with the scope and feeling satisfied with the results, is it really worth it to have this feature for a first time purchase? Thanks in advance!


texts: Cosmic microwave background radiation: why did it take so long to get here?

texts:
I understand that the universe after the big bang was a super dense cloud of elementary particles, and because of this high density was "not transparent" to light.
Then, roughly 400.000 years (380.000 to be exact) after density and temperature lower enough for the first hydrogen atoms to form. Thanks to the lower density, now photons were free to travel. Theoretically the Cosmic Microwave Background Radiation (CMBR) should be this light that "became free" 400.000 years after big bang.
Let's reason in 2d for simplicity. If the universe was a circle, the maximum radius could have been a bit less than 400.000 light years (it's probably an exaggeration, but let's simply assume that matter can't travel faster than light). So, if we imagine that the matter that eventually generated the earth was on the border of this circle (and somehow we are still close), the most far photon at that time should have been at 800.000 light years.

If my reasoning is correct, I can not understand how it is possible that we see that photon after 13 billion years. It should have reached us long long time ago.


texts: Why does kerr black holes have a ring shaped singularity rather than point?

texts: I read about ring singularity on Wikipedia and didn't quite understand the flow of liquids inside a rotating body causing a ring shaped singularity with zero thickness. Moreover is it specifically ring shaped and not torus shaped in any case?


texts:
According to the theory (or my understanding of it), the universe is not only expanding, but speeding up.

If the galaxies are moving apart, are the solar systems within them also moving apart from one another (& the planets within them), if they are, won't the Sun be moving away from the Earth?

Does the expansion of the universe occur at all scales or just large (inter-galactic) scales?


texts: Is there any residual oscillation left from the Moon rotation?

texts:
The Moon is tidally locked to Earth - a long time ago it was spinning, but after a time, the spin stopped - two factors stabilizing it: unequal mass distribution creating an equilibrium position where there's one "preferred" axis by which it faces Earth - an attractor, and the dynamic tidal forces acting as stress upon its structure, dissipating the energy - a friction, reducing the rotation speed.

So, at one point in time, it would no longer complete a full spin, but instead reverse its turn direction, start spinning in opposite direction, with maximum spin speed around the time where the "preferred" axis faces Earth, then slowing down again, until its spin stops, and reverses direction again - each time the angle smaller as the dynamic forces act against the direction of rotation - but also, with the rotation angle and angular velocity dropping, the value of the dynamic forces dropping.

There is a significant apparent wobble caused by the orbit's eccentricity, and possibly some actual continuous wobble, as the result - with the attractor axis missing Earth as result. But is there any long-term oscillation left over from the times when the Moon was spinning - a harmonic motion around its original spin axis, that would likely not be in tune with the orbital period? (I imagine the period of these oscillations would be very long; after all it's about reversing Moon's spin back and forth, by a force that isn't all that strong.)


texts: Why the earth orbits the sun from X to -X and not the opposite

texts:
I wonder all the time, why the earth orbits the sun as expected from east to west(X to -X) ?
It could be different, for example from the west towards the east .
What would happen if that were the case?


texts: Is the universe heading towards a Big Freeze, a Big Rip, a Big Crunch or a Big Bounce? Or is it part of an infinitely recurring cyclic model?


texts:
I'm well aware of the celestial bodies considered as the oldest visible in our galaxy.

My question might feel a bit odd: What is the oldest thing in the universe? I am puzzled with this question as the only answer that I have found by myself sounds weird.

As the notion of "old" only makes sense in an observable and ellapsing timeline, the most obvious answer to me would be "Time is the oldest thing". There is no such thing as a measurement of "old" until there is actually mass to allow time, so which predates which? I'm pretty sure the answer will tightly be linked to Planck's constant, but how?

What am I doing wrong? How could I find relief in an answer that wouldn't be just philosophical, but also logical? :)


texts: How dense would planet earth have to be to have the same gravity as Jupiter?

texts:
I was reading this question about how small could a planet be while having earth-like gravitational pull.

This got me thinking, how dense would planet Earth have to be to have the same gravitational pull as Jupiter while all the other factors staying the same (even if it is impossible in the real world)?

If there are any formulas could you please explain them so I understand them please?

Edit

Sorry about some of the confusion but I meant keeping everything but the mass the same.


texts: At what gravity does a body's gravitational lensing become observable?

texts: I wonder whether we know at how much gravitation a gravitational lensing around a celestial body becomes observable (visible in the sense of that we see stars on the wrong places, stars behind the body etc, similar like around this neutron star, but of course by far not as strong, I'd like to know where the boundary lies). Such visible gravitational lensing occurs around neutron stars, black holes and white dwarfs, right? What about red and brown dwarfs? They're also quite dense and have a high surface gravity (though not as high as the other mentioned bodies of course). Do they curve the space around them visibly?


texts:
We are having a problem with our scopes on cold nights.  (-10F is about average here.) One poorly aimed breath means that the eyepiece lens fogs.  We have to remove the lens and keep it under our arms to warm up to clear it.
The scopes are wrapped with heating tapes which does seem to keep the large lens from fogging/steaming up.    It is the eyepiece lenses that we are having issues with.
I searched Orion & Celestron's websites, but they just offer heating tapes which we already have.  I am concerned that frequently wiping the lens free of moisture, even with a soft cloth, could eventually scratch the lens.
Does anyone have a solution?


texts: Why are solar systems stable and not chaotic?

texts:
I am not trained in applied mathematics and am asking as a layperson.
It seems that the motions of the planets can be reliably predicted for thousands of years. Yet, if we consider a relatively simple system, the compound pendulum for example, its motion is highly unpredictable.
Chaotic pendulum - guess when it will stop flipping
The pendulum is highly constrained by its pivot points, therefore its components do not escape from the system. However, with a solar system, there is no rigid attachment and it seems intuitive that planets could randomly fly off into space as a result of chaotic orbits.
Question
Is our solar system inherently stable or unstable?
If unstable then how did it hang together long enough to form in the first place?
If stable, how can stability and predictability be consistent with many bodies moving relatively to one another?


texts: Can a liquid be used to de orbit debris?

texts: Can a liquid be used to slow a satellite to loose orbit by adding or taking momentum from a satellite, a type of devise that releases a liquid that expands and solidified adding surface area making more drag and or by adding weight?


texts: Does the Sun and Moon basically appear to "move on the sky" identically regardless of location and date?

texts:
For a long time, I've been trying to use Stellarium (simulation) and many visualizations and animations in order to get a good idea of how the Sun and Moon move across the sky, as seen by a human from the surface of Earth.
It's surprisingly difficult to grasp this, at least for me.
It seems to me that the Sun always "rises" in the "East", goes slowly up to "above our heads", and then sets to the "West" in the same manner, making a kind of half-circular path. After that, after some unknown time, the Moon kind of "follows" the Sun in this (more or less) exact path, also coming up from the East and "setting" in the West.
The fact that the Moon is orbiting around Earth, and Earth orbits around the Sun, and yet they move like this, makes no sense to my brain. It seems like the described path would only be true for the Sun, or the Moon, but not both.
And is it always (roughly) the same? Or does this vary depending on where you stand on Earth, and what date/year it is? It was very difficult for me to make my own experiments in Stellarium. It seemed like it would vary a bit, but not too much.
The reason I'm trying to figure this out is both because of general curiosity, and wanting to accurately describe the Sun and Moon in my story. In The Lord of the Rings, Tolkien frequently talks about the movement and look of these objects, which appear to follow the same rules as in our world. It's clear that he had very deep knowledge of how these things appear to move on the sky over the day, and I like such "details" to be accurate.
Basically all the animations and illustrations I find of the movement of the Moon and Earth around the Sun are focused on a "space overview" rather than how it looks from Earth.


texts: Relationship between Hubble paramater and absolute magnitude

texts: I have a set of data that is absolute magnitude of a star and its redshift. Is there any way to compute the value of the hubble parameter given that magnitude and redshift?


texts: Where should I be looking to see Neowise in southern Sweden?

texts:
I'm in the south of Sweden and just spent over an hour trying to find Neowise, but didn't see it. I live in a rural area, there are no lights nearby and there is a small hill with clear visibility between north and northeast. I went out at 00:45 (GMT+2) and came back in at 02:00, not having seen anything resembling a comet, not even with binoculars.
News outlets around here are saying everything from "Look towards the Big Dipper and you'll see it" to "Between north and north-east", or "To the west in the evening and east in the morning". Many say that it would be somewhere "down to the left of Capella". Frankly it's quite frustrating to not be able to find neither Neowise nor a surefire description of how to locate it.
This photo is taken to the north-east around 01:45 (GMT+2). Where in relation to the stars on the photo would Neowise be? The brightest one is Capella according to Sky Maps.


texts: What is the maximum possible separation between sun and moon in the earth sky for an eclipse to occur?

texts:
How do we calculate the maximum angular separation between sun and moon during:

Solar Eclipse
Lunar Eclipse


texts: Is there any hard evidence that rogue planets exist?

texts: A rogue planet (or a rogue anything, for that matter: a celestial body other than a star) is something that's drifting through space without being attached by gravity to any star. They're just out there in the big black void. Is there any evidence that they actually exist? Or are they purely hypothetical?


texts: What is the evidence that galaxies rotate?

texts:
Do all galaxies rotate?  If so, do they rotate in the same direction?

What observations have been made of galaxy rotation?


texts: Is there any man-made satellite orbiting our moon?

texts: I am just wondering whether any man-made satellites are currently orbiting our Moon. If there is one, would I be able to see it using my telescope?


texts:
The Saha Equation:

$$\frac{N_{i+1}}{N_{i}} = \frac{g_{i+1}}{g_{i}}\frac{g_e}{N_e h^3}(2 \pi m_{e} kT)^{3/2} \exp(- \chi_i / kT)$$

where $\chi_i$ is ionization potential. My problem is about the sign ($+$ or $-$) of $\chi_i$. I know that its formula is something like:

$$\chi_i = (13.6\ \mathrm{eV}) Z^2 \left(\frac{1}{n_i^2} - \frac{1}{n_j^2}\right)$$

and in this case $n_j$ is $\infty$. But I get confused about it's sign ($+$ or $-$). Can someone simply say what is the sign of the formula for ionization potential and the sign of the formula for the Energy we need to go from $n_i$ to $n_j$.

I mean: What is the complete formula for $E_m - E_n$ and also $\chi_{n}$? (with the right sign so they can be used in the first Saha Equation above)


texts: Could we detect an earth-like planet around another star?

texts:
Could we detect an earth-like planet (e.g. similar distance from star, and similar size) with current planet finding techniques?  If an earth-like planet were orbiting our nearest star, excluding the sun :-), would we have detected it by now?

It seems current techniques tend to find large planets orbiting close to their star, or for direct observation, find large planets further away from their star.  Likewise, we have found many very large planets very close to their star (e.g. hot Jupiters), and some rocky planets somewhat larger than earth close to stars, but not very many (if any) that are very similar to earth (e.g. distance from star and size).  It seems the current techniques available do not readily detect earth-like planets though, so it may be that there really are many earth-like planets, but we haven't seen them yet.  Here is an article on current planet detecting techniques.

http://en.wikipedia.org/wiki/Methods_of_detecting_exoplanets


texts: How spectrographs that measure radial velocities manage to translate variations in the stars' spectrum lines into the "speed" of the star?

texts: Like ESPRESSO or CARMENES, for example. I just don't understand the process or the way these spectrographs manage to obtain the velocity of the "wobble" of the star (due to the presence of another companion such as an exoplanet) just by analyzing variations on spectral lines.


texts: I read about natural gravitational eddies that travel in a wave that black holes have. They also have a strong magnetic field. Does those eddies follow magnetic field lines of the rotating black hole?  Can a galaxies have their own gravitational eddies like a black hole does?


texts: Why does exoplanets' naming start from "b" and not from "a"?

texts:
An exoplanet named after its host star receives the suffix letter "b" if it was the first planet discovered to orbit that star, or "c" if it was the second, or "d" if it was the third, and so on.

Why this naming system starts from "b" and not from "a"? Is the letter "a" reserved for another object in the planetary system? If yes, for what is "a" reserved?

As an example, here is a list of the major objects in the Gliese 667 triple-star system:

Gliese 667A (a star)
Gliese 667B (a star)
Gliese 667C (another star)
Gliese 667Cb (a planet orbiting star C)
Gliese 667Cc (another planet orbiting star C)


texts: Is there any legitimacy to this report of planet nine being discovered by an amateur?

texts: Saw this article on someone from Hampshire UK reporting a possible discovery of Planet Nine. I thought that it would be too dim for anything but the most powerful telescopes to see. Is there any legitimacy to this article?


texts: calculating paralax angle from observations

texts: Say I have 2 records, in horizontal coordinates of the azimuth ($A_z,A_z^{'}$) and altitude ($\alpha,\alpha^{'}$) of the moon at the same time in two different places that are $\theta$ degrees apart (taking the angle from the centre of the earth and assuming the earth is a perfect sphere). Is it possible to calculate the parallax angle of the moon between these two points with only the values above? if so how and if not what other values would I need?


texts: What are these data in light curve generator

texts:
I'm generating light curves for some variable stars on the AAVSO website (see https://www.aavso.org/lcg ) . I can choose what type of data must be plotted. Can you explain what they are?

Thank you in advance!


texts: I'm doing an Astronomy project and need to terraform a moon. It is about the size of Mars and orbits a gas giant which is at a distance of 1.28 AU of its star. I though about putting a sunshade at the Lagrange point L1 between the planet and the star, but I don't think that would be consistently cooling the moon (by about 30K). For the purpose of this project, it's surface is mostly water, and the atmosphere is mostly water vapor. Any ideas how to tackle this problem? Any way to raise the albedo, or maybe getting a sunshade to work? Be it at a Lagrange point or in orbit around the moon? I still want to do this project by myself, I could just use an idea or two, maybe some initial help with the calculations. Thanks in advance :)


texts: Why do linear velocity redshifts correspond to linear pixel shifts when the spectra are binned in constant log wavelength?

texts:
In Tonry & Davis (1979), they describe spectroscopic redshift measurement via correlating with templates at known redshift. In Section IIIa, they say "Because the spectra are binned linearly with ln$\lambda$, a velocity redshift is a uniform linear shift.  Why is this the case?

Here is the relevant paragraph from the paper:


texts: What makes protoplanetary disks start rotating? (Initial energy needed to rotate)

texts:
Planets form from a protoplanetary disk that has been rotating around its star. The initial energy that makes them rotate really matters to me.

Why did the protoplanetary disk start rotating around the star?
Where did the initial energy to rotate come from?
Why hasn't all the disk material been swallowed by the star?


texts: Is it possible to define a state of zero absolute angular momentum anywhere in the universe?

texts: Conservation of angular momentum doesn't seem correct to me. I think this because radiation is present everywhere, no space is completely void, and no object perfect. Thus all objects will have uneven radiation pressure. My question is there anything in space that is not increasing or decreasing in rotation or is fixed in space? Is the rate in which an object spins is determined by surrounding celestial bodies? Or in other words any object in space depending on its position in the galaxy will slow or increase to an ideal rotation?


texts: How do we know background radiation does not come from undiscovered galaxies?

texts:
Each year while observing voids, astronomers discover more and more unexpected galaxies within them.

How do we know the cosmic microwave background radiation is not the weak light from distant galaxies?
How do we know cosmic microwave background radiation is not stars' light reflected by dark objects?


texts: How are the atomic clocks synchronised between worldwide VLBI telescopes?

texts:
I know that VLBI needs precise synchronised atomic clocks to calculate the baseline distance between the radio telescopes at cm level. But i cant´t find any information how the atomic clocks are kept synchronised over long distances (several thousand kilometres around the world). As far as i know GPS is not accurate enough.... How are atomic clocks kept synchronised on all VLBI telescopes ?

Thanks a lot :)


texts:
In the Big Bang model of the Universe, every observable thing is thought to have expanded very abruptly from a point of infinite density and zero volume. However, the problem with this assertion is that for something to expand it first needs a locality in which to do such, which brings us to my question. How can scientists believe that "everything" was condensed and then suddenly expanded, since such is automatically an oxymoron?

Expansion is a physical property of the universe and yet if "everything" had no where to expand, how could it have expanded? There was "nothing," was there not? Even if we drop the big bang model and talk about "everything's" being in a singularity point, the question still stands.

Where was the singularity point? Asked differently, into what is everything expanding (which is a rhetorical question I ask to implicate space-time as necessarily existing irrespective of any theoretical expansion and likely so vastly that one might as well assume that space-time is infinitely large).

As well, it seems counter-intuitive, as little impressive a word as that is given its context here, that something like... well, "everything" could be condensed in the first place, let alone infinitely condensed and with zero volume. When thinking about the big-bang model, I can't help but conclude that a mere analogy was originally used for stellar birth, evolution and finally explosion. The issue though is still extension.

As far as I understood it, the big bang model states that space itself was also condensed along with everything else in the observable universe to a point of singularity. So, if that's the case, then [where] was the singularity point?

It ends up becoming an infinite regress, of course. Or, if you don't like the issue of "time" presupposed by my question, then look at the question after the universe started expanding. All the matter had to expand somewhere and "somewhere" is not "nothing." - It just seems that the model is somehow overlooking said feature of the actual universe and trying to account for it by mere assertion.

If you want to call it space-time, extension, space, where everything supposedly expanded to and is and is still expanding to, then that's my question. How does the theory allege where everything is yet at the same time assert that prior to the singularity point, there was literally nothing.


texts:
Would it be possible for two planets orbiting around each other to share a moon that orbits both separately? Would the moon in question would do something akin to a large figure 8 as it orbited both or would this be possible in the first place?

Also, what effects might we see on the planets because of this?

(Side note: This stems from a Worldbuilding question, but I figured I would ask the astronomy question on the Astronomy StackExchange)


texts: Is there a way to know the time according the moon appearance?

texts: Is it possible know the time according to the moon location? To look at the sky at night and then determine what the time is.


texts:
The GIF below is copied from my question earlier What might a CN filter be in the context of comet watching? Is it showing dust, or gas, or something else? where I'd said:

In this post on the website for the Comet Wirtanen Observing Campaign at wirtanen.astro.umd.edu, there's mention of a CN filter being used to observe the comet, and this cool GIF.

It certainly looks like the comet is spinning; it reminds me of images of the density of the solar wind as active areas on the sun "spray" out matter as the Sun rotates, or the patterns discussed in Why is one of these two concurrent fuel-dump spirals blue?.
Question: This leeds me to wonder if I'm just imagining things, or if some comets really do spin? If so, how fast have they been observed to rotate?


texts:
I'm a software developer and I'm developing a planetarium and the first thing that I don't know is: What is the shape of the sky?

First, I though that it is a semi-sphere, but searching on Internet I have found a lot of youtube tutorials, and all of them draw a semi-sphere but flatten (you can search skydome on Google and find a lot of tutorials).

My question is, how can I represent sky on a virtual 3D world? How much do I need to flat a sphere to get a real sky representation?


texts: Unable to locate Big Dipper through Polar Axis Finder

texts:
I have recently purchased a CG-4 Polar Axis Finder for my CG-4 mount.

This PAF has markings on the lens for aligning with Big Dipper or Cassiopeia, which looks like this:

I have aligned the PAF to the mount during the day, and I am now attempting to use it at night. My understanding is that I need to rotate the mount in RA to match the image in the PAF to the actual stars in either Big Dipper or Cassiopeia. This will give me the correct position to then re-target at Polaris to find NCP.

I have been trying for 2 nights, and still no luck! The Big Dipper and Cassiopeia are in front of me - I can see them very clearly with the naked eye. Yet when I look through the PAF, I cannot see any star patterns resembling either of my targets. There are stars, but it's like the magnification is too high, maybe? No stars forms the pattern I am looking for, in any orientation. I cannot derive the connection between what I see with the naked eye right in front of me, and what I see through the PAF. I understand that the image in the PAF will be mirrored vertically and horizontally - still no luck. I can easily find my target through the Finder Scope on my main tube, but not through the PAF.

I have devised an experiment. I have positioned my tripod under a power line running high above me. I have aligned my view at Alkaid (western tip of the Big Dipper handle) with the wire, so I can see it in the PAF. Good news - I can see it in the PAF, and I can see Alkaid (it's the brightest in the area, so I am fairly sure that I can use it as a reference point between what I see with the naked eye and what I see through the PAF).

Bad news - no matter how I rotate my mount in RA while keeping the handle tip aligned with Alkaid, there's nothing that matches the rest of Big Dipper...

What is going on? I can't even explain this with a
"Defective PAF Theory"...


texts:
Maybe its a dumb question, but to convert Jy/beam to Jy, I just have to multiply it by the beam size in sr right?

Being $\Omega$ the beam size:
$\Omega = \frac{\pi \theta_{maj} ~~ \theta_{min}}{4 \ln{2}}$

Jy/beam $\cdot \ \Omega$ = Jy   ?


texts: Why there are less massive stars than low mass stars?

texts:
What factor determines the selection of star mass?

And how we know that the number of massive stars is less than the number of low mass stars?


texts: How can the radius of a star be measured using parallax?

texts:
The radius of Betelgeuse was measured using parallax. From the Wikipedia article:

  the estimated parallax from the Hipparcos mission was 7.63±1.64 mas, yielding an estimated radius for Betelgeuse of 3.6 AU.

I understand how a distance can be determined using parallax, but I don't understand how the radius can.

What is the math behind deriving a radius from a parallax measurement?


texts: Are there other proofs of the expanding universe apart from the redshift?

texts:
The theory of the expanding universe is so widely accepted, that the redshift is sometimes used as a measure of distance to far away galaxies.

But is it still possible that the redshift is caused by some unknown phenomena and not by galaxies moving away from each other?

Is there any other proof (apart from the redshift) that the universe is indeed expanding and far away galaxies are moving away from us?


texts: Local atmospheric pressure and surface gravity in Jezero crater?

texts: The Jezero crater on Mars is the target location of the American Mars 2020 mission. I can't find any good values on the local air pressure and gravity, though the elevation is around what is considered the mean sea level so I suppose the atmospheric pressure is average, around 0.006 atm (0.088 psi). The Ingenuity helicopter reportedly was tested at that pressure. Does anyone know the exact local pressure at Jezero crater's bottom and the local surface gravity? Thank you.


texts: The Luminosity of two stars with same brightness but at different PC away

texts: Stars A and B are both equally bright as seen from Earth, but A is 120 pc away while B is 24 pc away. Which star has the greater luminosity? How many times brighter is it?


texts: What apparent sizes of the sun an Earth like planet can have?

texts:
What is the smallest and largest sun an Earth like planet could have in its sky? What kinds of stars are those?

I saw this video and thought, yeah, some of those stars are really big in absolute terms, but Earth would have to be orbiting them proportionally further to keep the climate, making them smaller in the sky, so the video is not accurate.


texts: What is the difference between ICRS and ICRS2?

texts:
When I first learned about Positional Astronomy (50 years ago), everything was described with respect to the Equator and Ecliptic (true or mean, of date or of a specific epoch). Then 'they' invented the International Celestial Reference System (ICRS). An inertial reference system, so true and mean no longer apply and neither does it require an epoch to be given.

Now I see things are referred to the International Celestial Reference System Version 2 (ICRS2).

What changed?


texts: How large does refraction become in radioastronomy?

texts:
For atmospheric refraction of visible light, Wikipedia gives the order of 1 arc minute at 45° altitude above the horizon, and 5.3 arc minutes at 10°. This is caused by the dielectric polarizability of all of the bound electrons in all the atoms of the atmosphere.

At the much lower HF frequencies of radio, the free electrons and ions will contribute, and some forms of radio communication have relied on refraction at large incident angles to deflect terrestrial signals back to the Earth at a distant ground station.

So I expect that at the lower frequencies used in radio astronomy, corrections to the observed location of radio sources due to ionospheric refraction could be much larger than those at visible wavelengths, but I am not sure.

How large does can this effect ever get? At what frequency? Are there ever corrections as large as 1 degree?

I started thinking about this after asking How many stations could one hear with an AM/FM radio in front of the ISS' cupola window? which includes the image below.

below: from the Radio Jove Project's exercise The Effects of Earth's Upper Atmosphere on Radio Signals.


texts: Was the Crab Nebula to the southeast of Zeta Tauri 1,000 years ago?

texts:
I've been reading about SN 1054 and how most professional investigators believe that it is the Crab Nebula today. But the wiki article has this to say:

  The location of the guest star as "to the south-east of Tianguan, perhaps several inches away" has perplexed modern astronomers, because the Crab Nebula is not situated in the south-east, but to the north-west of Zeta Tauri.

Well, then I remembered the precession of the equinoxes. I know precession does not change the fixed background of stars, but can change their relative location.

If we rewind the precession 1000 years, does Zeta Tauri cross the zero meridian or the celestial equator?


texts: This question sounds like silly and rather fantasy filled. But can our earth's gravity capture a sizable asteroid and make it rotate around our earth like our moon which become visible from ground. Is there any theoretical possibility or practical probability?


texts: Does the KBO 2014-MU69 have two numbers and entries in asteroid databases? How did it get promoted to Major Body designation?

texts:
In this answer I show that the (now pretty famous) Kuiper Belt Object 2014-MU69 has two entries in JPL's ephemeris generating Horizons site; Major Body 3713011 and Small Body 486958. I'm pretty sure these represent the same body, their trajectories differ by about a half-million kilometers only, and that difference is probably only because for one of them the trajectory is only based on a 118 day arc of it's ~100,000 day orbit at ~43 AU back in 2015.

Are both 3713011 and 486958 still official references to the same body, but in different catalogs or databases? Is there a disambiguation page linking the two numbers somewhere? How did 2014-MU69 (unofficially Ultima Thule) get promoted from SB to MB?


texts: I am curious that there are lots of celestial objects (planets, comets, stars, galaxies, etc.) in space. How do we name them?


texts:
As I understand it, objects falling into an old/inactive/dead/non-pulsar neutron star would be ripped apart, form an accretion disc and eventually emit a burst of radiation as they fall onto the surface of the star.

If this happened to:

a comet
a dwarf planet
an ice giant

...roughly how much energy/radiation would each produce? Also, if possible, over what kind of time frame? I'm only looking for ballpark figures in all cases.


texts: Why are radar images of comets shaded only on one side?

texts:
NASA published a picture of a comet the other day. The image shows the comet being lit from above. See NASA's picture below.

However, since it is a radar image, I would have assumed to get a shading on all sides and grazing angles, like velvet or scanning electron microscopy. Or alternatively to have the sides facing the radar dish being shaded, and the edges being dim. For the velvety look, compare to this picture from Wikipedia:

So why is the comet's image shaded only from above?


texts: Is there a limit to the size of a black hole?

texts:
The mass of a star is limited to about a couple of hundred solar masses. The fusion rate scales strongly with density (which is why the most massive stars have extremely short lifetimes) so if a star was massive enough (above the Eddington limit) then the radiation pressure would blow it apart.

Now a black hole also has radiation, but by definition it is not capable of escaping due to gravity. But is there in some way a limit of the size. Imagine that all the matter in the universe formed a black hole. Should that be possible or is there a law which forbids creating it?


texts: Can any stars ever form supermassive black holes?

texts: I heard that most supermassive blackholes are not formed by stars – in fact, we aren't even sure how they're formed. But could a star, with enough mass and low enough metallicity, form a supermassive black hole? Rather, would there be any stars that fit this category?


texts: Solar System formation, considering its and the universe's age

texts:
It is known that the Sun is 4.6 billion years old, and the complete Solar System is of a similar age. The class of stars to which the Sun belongs seems to be quite common. Stars of its class can live for 8 or 9 billion years before moving to the red giant stage. Also, the whole universe is less than 14 billion years old.

Thus, not many generations of Sun-like stars could have existed before the first stages of our Solar System. And heavy elements are created when stars collapse. While there could be a lot of star classes with much shorter lifespans, I'm wondering: where do the heavier elements came from? Not only for our Solar System, but many others. I think it is very likely that our Solar System is of average age; do the numbers match between dead stars and remote stellar systems? Are there studies about this?


texts:
Is the calculated age of the universe that of the visible universe or the entire universe? I dont know how the age is calculated but if it is believed that all we see visibly IS the entire universe then that means it is finite. If it is not finite then the light from the really far astronomcical objects has not reached us yet so there is a possibility it is infinite.
Can the method used to calculate the universe age since the Big Bang guarantee that it has an age or is it possible that it existed infinitely?


texts: Calculating Field of View of a Telescope Given Time of Another Object

texts: Given the celestial coordinates of a certain star and the time it takes to pass across your telescope's eyepiece, how can you calculate the field of view of the telescope?


texts: How big should be a piece of neutron star to reach Earth's mass?

texts:
Imagine a neutron star could be broke into pieces.

How big would be the piece matching Earth's mass?


texts: I have seen so many articles but how can finding exoplanet density determine composition

texts: I have seen some articles and lectures that state but i cant seem to put my finger on on how does density affect composition and how are you able to determine composition from exoplanet density.